Microsoft Corp. and two law firms are among the tenants who have eyed Boston Properties’ 40-story office tower project in Manhattan, which passed a milestone last week on its way to becoming the first new Midtown office building in three years. The 1.1 million square foot tower, known as 250 W. 55th St., was “topped out” last week, meaning the last beam was placed at the top of the building.

Now Boston Properties is hoping to lease it up before its scheduled opening in 2014. About 20% of its space already is spoken for by law firm Morrison Foerster. Microsoft would be a great get. The software giant is looking for space for its New York office, which is now on Sixth Avenue.

Microsoft also is looking at other possibilities besides 250 W. 55th St., including 51 Astor Place, according to people familiar with the matter. At the same time Boston Properties has danced with many prospective tenants.